It doesn't really matter for this post WHAT causes Autism, but it matters that the link isn't solid. The fallacy here is assuming that because two things exist in close proximity that one must be the cause of the other.
A hypothetical example would be that the majority of mass murders happen in the South (I made that up and don't know if that is even remotely true), therefore since the South is warmer with more humidity, the people are more likely to kill. Well, it may be warmer and more humid, which can certainly make people miserable, but that doesn't mean that heat/ humidity creates killers. There are other factors to consider. Do these Southern mass murderers have other things in common? Race, socioeconomic status, gender, psychopathy, sociopathy? What caused the psychopathy/ sociopathy? Was that genetic or due to the environment of the killers? Simply assuming two things are related doesn't make them so. They may happen in the same span of time or with the same frequency, but that does not mean that one causes the other.
Often I see this with the violence debate. People often debate that violent video games cause violent behavior, but many studies have shown just the opposite. Here, here, here, and here are articles in mass media that contradict that. I know you can find many articles that say it's true, but it really isn't clear, and is often a false cause. We are linking two things together that aren't necessarily related. Gun violence isn't necessarily related to video game violence. If it were we would have more issues throughout the world. Many other countries actually spend more on violent video games and play them more frequently, but are not seeing mass murders. It may be the cause. I don't know. What I do know is current research doesn't support this, and we can't assume that because two things happen at the same time, or even together, that one causes the other.
That is the false cause fallacy or the fallacy of cause.